Liquid crystal displays (LCDs) have been used in not only small display devices such as the display of a cellphone but also TV sets with a big screen. Conventional TN-mode LCDs have relatively narrow viewing angles. Recently, however, the viewing angles have been broadened in LCDs of various modes.
As one of such LCDs with a wider viewing angle than a TN-mode LCD, known is an OCB (optical compensated birefringence) mode LCD. FIG. 22 is a schematic representation illustrating an ordinary OCB mode LCD 600A. In FIG. 22, liquid crystal molecules in a liquid crystal layer 900 are in bend alignment state.
In this LCD 600A, each of two alignment films 720 and 820 that face each other with the liquid crystal layer 900 interposed between them is subjected to a rubbing treatment in direction, thereby defining the pretilt direction of liquid crystal molecules in the liquid crystal layer 900. In the liquid crystal layer 900, some liquid crystal molecules 902a, which are closer to the active-matrix substrate 700, tilt toward the +x direction to the viewer's eye, while other liquid crystal molecules 902b, which are closer to the counter substrate 800, tilt toward the −x direction to his or her eye. In this manner, those two groups of liquid crystal molecules tilt in substantially antiparallel directions as viewed in the thickness direction. As a result, a variation in refractive index, which would otherwise be caused due to varying viewing angles in the x direction, can be compensated for, thus realizing a wide viewing angle.
Meanwhile, the alignment directions of liquid crystal molecules in the liquid crystal layer 900 will change according to the magnitude of the applied voltage, thus varying the effective retardation of the liquid crystal layer and eventually changing the transmittance of the liquid crystal layer 900. The rate of this change is as fast as only several ms, and therefore, the OCB mode is known as a mode that would realize response at high speeds. An OCB-mode LCD achieves a wide viewing angle as described above. To further broaden its viewing angle, however, it has lately been proposed that an alignment division be made by performing a rubbing treatment in multiple different directions (see Patent Document No. 1, for example).
FIG. 23 is a schematic cross-sectional view illustrating a one pixel region of the LCD 600A1 disclosed in Patent Document No. 1. The LCD 600A1 has Domain A in which the alignment films 720 and 820 are subjected to a rubbing treatment in +x direction and Domain B in which the alignment films 720 and 820 are subjected to a rubbing treatment in direction. To define multiple regions with mutually different alignment directions in a single unit region in this manner is called “alignment division”. By providing these two types of domains with mutually different rubbing directions for the LCD 600A1, the variation in refractive index can be compensated for in the x and y directions and the viewing angle can be further broadened as a result.
On the other hand, the viewing angle has also been broadened in LCDs operating in non-OCB modes. For example, some VA (vertical alignment) mode LCD that would achieve a high contrast ratio is known to get such alignment division done by providing ribs and/or slits for two electrodes that face each other with a liquid crystal layer interposed between them (see Patent Document No. 2, for example). Such a mode is sometimes called an MVA (multi-domain vertical alignment) mode.
An LCD 600B as disclosed in Patent Document No. 2 will be described with reference to FIGS. 24(a) and 24(b), which are respectively a schematic plan view and a schematic cross-sectional view of the LCD 600B.
In the LCD 600B, each pixel is split into two subpixels SP-A and SP-B, which are defined by subpixel electrodes 710a and 710b. By producing mutually different potentials at those subpixel electrodes 710a and 710b when a grayscale tone is displayed, the viewing angle dependence of the γ characteristic can be reduced. To split a single pixel into two or more subpixels in this manner is sometimes called a “pixel division”. Furthermore, in this LCD 600B, the polarities of the voltages applied to the subpixel electrodes 710a and 710b are inverted every vertical scanning period, for example, thereby reducing the image persistence phenomenon.
Also, in this LCD 600B, a slit (an opening) 712a, 712b is provided in the subpixel electrodes 710a and 710b of the active-matrix substrate 700, while the counter electrode 810 of the counter substrate 800 has ribs (projections) 812a, 812b. These slits 712a and 712b and ribs 812a and 812b run in two intersecting directions D1 and D2. When a voltage is applied to the liquid crystal layer, the liquid crystal molecules will align themselves from the ribs 812a and 812b toward the slits 712a and 712b, thereby forming multiple liquid crystal domains A, B, C and D with mutually different alignment directions in each of the subpixels SP-A and SP-B. Consequently, this LCD 600B realizes a symmetric viewing angle characteristic.
Unlike a TN- or OCB-mode LCD in which the pretilt direction of liquid crystal molecules is defined by alignment films, those linear slits and/or ribs apply alignment regulating force to the liquid crystal molecules of an MVA-mode LCD. That is why the alignment regulating force applied to the liquid crystal molecules within a pixel region varies according to the distance from a slit or a rib, thus making the response speeds of those liquid crystal molecules different within the single pixel region. On top of that, in the MVA-mode LCD, a region with a slit or a rib has a decreased transmittance, and will cause a lower display luminance. To overcome these problems, it has been proposed that a mode that forms an alignment division structure using alignment films that define pretilt directions be applied to even such a VA-mode LCD. Such a mode is called a VATN (vertical alignment twisted nematic) mode. In a VATN-mode LCD, for each of two alignment films that face each other with a liquid crystal layer interposed between them, provided are two sets of alignment regions that define two different pretilt directions for the liquid crystal molecules (see Patent Document No. 3, for example).
Hereinafter, the LCD 600C disclosed in Patent Document No. 3 will be described with reference to FIGS. 25 and 26. Specifically, FIG. 25(a) is a schematic representation illustrating the alignment treatment direction of the alignment film 720 of the LCD 600C, while FIG. 25(b) is a schematic representation illustrating the alignment treatment direction of the alignment film 820. As shown in FIG. 25, the circular cylindrical liquid crystal molecules are tilted so that their (substantially circular) end faces the viewer. In the liquid crystal layer of this LCD 600C, four groups of liquid crystal domains A, B, C and D are produced by combining the two alignment regions of the alignment film 720 with those of the alignment film 820. FIG. 25(c) illustrates the alignment directions of liquid crystal molecules around the center of the liquid crystal domains A through D. Supposing the alignment directions of the liquid crystal molecules around the center of the liquid crystal domains A through D will be referred to herein as “reference alignment directions” and the azimuthal components of the reference alignment directions will be referred to herein as “reference alignment azimuths”, the liquid crystal domains A through D have mutually different reference alignment azimuths, thus realizing a symmetric viewing angle characteristic.
FIG. 26 is a schematic plan view of the LCD 600C, in which a pixel division structure is formed in the same way as in the LCD 6008 and four liquid crystal domains A through D are defined in each subpixel SP-A. With such a pixel division structure, the LCD 600C has a reduced degree of viewing angle dependence of the γ characteristic.
The VATN-mode LCD 600C will sometimes produce peculiar misalignment. As a result, there is a region, of which the luminance is lower than the one associated with a grayscale tone to be displayed when the screen is viewed straight, thus producing dark lines. Those dark lines will be produced not only at the center of the subpixel electrode 710a where the liquid crystal domains are adjacent to each other but also along at least some of the edges of the subpixel electrode 710a as well. If in any of the edge portions of the subpixel electrode 710a associated with the liquid crystal domains, the azimuthal direction that intersects with that edge portion at right angles and that points toward the inside of the subpixel electrode 710a forms an angle of greater than degrees with respect to any of the reference alignment directions of the liquid crystal domains, then a dark line will be produced inside of, and substantially parallel to, that edge portion of the subpixel electrode 710a. Such a dark line will be referred to herein as a “domain line”. FIG. 25(c) illustrates four domain lines DL1 through DL4. They say that those domain lines are produced because the reference alignment directions of the liquid crystal domains A through D and the directions of the alignment regulating forces produced by an oblique electric field generated at the edges of the subpixel electrode 710a have mutually opposing components, thus disturbing the alignment of the liquid crystal molecules there. Those domain lines are produced at various locations according to the alignment treatment directions of the alignment films 720 and 820. On the other hand, dark lines are also produced around the center of the subpixel electrode 710a because the liquid crystal molecules have mutually different alignment directions and do not transmit light at the boundaries between the liquid crystal domains A through D. Such dark lines CL around the center of the subpixel electrode 710a are sometimes called “disclination lines”.
The dark lines including domain lines and disclination lines, will look moving according to the viewing angle. And unless those dark lines are shielded from incoming light, the grayscale will be inverted. For that reason, in the LCD 600C, the dark lines are shielded from incoming light, thereby minimizing the decline in viewing angle characteristic. Specifically, the domain lines DL1 and DL3 are shielded with source bus lines S and the domain lines DL2 and DL4 are shielded with gate bus lines G and CS bus lines CS. The disclination lines CL are shielded with storage capacitor lines CS1 and drain lines 717.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-293308    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2004-62146    Patent Document No. 3: Pamphlet of PCT International Application Publication No. 2006/132369